To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the wells and the guns fired to create openings in a casing and to extend perforation tunnels into the surrounding formation.
The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.
To address these issues, pressure in a wellbore interval is manipulated in relation to the reservoir or surrounding formation pore pressure to achieve removal of debris from perforation tunnels. The pressure manipulation includes creating a transient underbalance condition (the wellbore pressure being lower than a formation pore pressure) prior to detonation of a detonation cord or shaped charges of limited energy. Pressure manipulation also includes creating an overbalance pressure condition (when the wellbore pressure is higher than the formation pore pressure) prior to detonation or explosion of shaped charges of a perforating gun or a propellant. Creation of an underbalance condition can be accomplished in a number of different ways, such as by use of a low pressure chamber that is opened to create the transient underbalance condition, the use of empty space in a perforating gun or tube to draw pressure into the gun right after firing of shaped charges, and other techniques. The underbalanced condition results in a suction force that will extract debris out of the perforation tunnels and fluid from the wellbore into the tube enabling the well to flow more effectively or more efficient injection of fluids into the surrounding formation. Creation of an overbalance condition can be accomplished by use of a propellant (which when detonated causes high pressure gas buildup), a pressurized chamber, or other techniques. The burning of the propellant can cause pressure to increase to a sufficiently high level to fracture the formation. The fracturing allows for better communication of reservoir fluids from the formation into the wellbore or the injection of fluids into the surrounding formation.
The manipulation of wellbore pressure conditions causes at least one of the following to be performed: (1) enhance transport of debris (such as sand, rock particles, etc.) from perforation tunnels; (2) achieve near-wellbore stimulation; and (3) perform fracturing of surrounding formation.
During the manipulation of pressure, one or more packers or plugs are known to be positioned between the inside of the wellbore and the outside of the perforating gun or tube to isolate the interval over which the detonation or explosion takes place to achieve a quicker and amplified response for the underbalance or overbalance effect.
It remains desirable to provide a device for confining the effects of a dynamic underbalance or dynamic overbalance in a defined region of the wellbore to enable removal of debris from the perforation tunnels and/or stimulation within the well.